1. Field of the Invention
This invention relates to the purification of sodium chloride brines and, more specifically, to an improved process for purification of sodium chloride brines containing strontium, calcium and magnesium impurities.
2. Description of the Prior Art
The production of chlorine by the electrolytic decomposition of sodium chloride brines is well known in the art. In order to maximize the efficiency of electrolytic cells, it is desirable to purify the brines prior to electrolysis to remove impurities such as strontium, calcium and magnesium ions which generally exist in raw brines. Thus, removing strontium impurities from brine fed to a mercury electrolytic cell has been found to minimize the formation of "mercury butter", a thick flowing or solid material which is generally formed on the surface of the flowing mercury cathode. See commonly assigned U.S. Pat. No. 3,954,580. Mercury butter is composed chiefly of mercury with small amounts of metallic impurities, mainly iron, and its presence in a mercury electrolytic cell is undesirable since it effects a decrease in current efficiency of the cell and increases the hydrogen content of the chlorine produced by the cell, thereby presenting a safety hazard and reducing the amount of chlorine that can be subsequently liquified from the gases so produced. In addition, purification of the mercury exiting the cell becomes more complex and costly as the amount of mercury butter increases, presenting hygene and pollution problems.
Removal of strontium, calcium and magnesium impurities also minimizes hydrogen evolution and anode wear in other types of electrolytic cells, e.g. diaphragm cells.
Melt refining processes, in which a sodium chloride melt is treated for removal of calcium, magnesium and sulfate impurities, such as that disclosed in U.S. Pat. No. 3,840,651 (issued in 1974 to D. T. Dreland), are uneconomical because of the high energy demands inherent in the process.
The process of Canadian Pat. No. 506,130 (issued in 1954 to D. W. Hengeren), in which sodium carbonate and sodium hydroxide are added to raw brine to form insoluble precipitates of magnesium hydroxide and calcium carbonate and in which the resultant brine and the suspended solids are passed over a bed of sodium chloride for removal of the precipitated impurities, has the disadvantage of requiring handling of large quantities of solid sodium chloride, in removing and replenishing the sodium chloride solids in the bed.
Of the processes which treat aqueous brines without use of such sodium chloride beds, those disclosed in U.S. Pat. Nos. 2,764,472 (issued in 1956 to W. R. Cady et al.); and 3,816,592 (issued in 1974 to F. Rinaldi) are typical. The Cady process treats brines containing dissolved calcium and magnesium chloride and calcium and strontium sulfate by introducing sodium sulfate to precipitate calcium sulfate, and treating the resulting liquid, after removing the calcium sulfate solids, with an alkali metal carbonate and either an alkali metal or alkali earth metal hydroxide to precipitate the carbonate salt of calcium and strontium and magnesium hydroxide. In the Rinaldi process raw brine containing impurities such as Ca, Sr, Ba, Mg, Fe, Al and Ni is mixed with a solution containing SO.sub.4.sup.-- ions for precipitation of sulfates of calcium and barium, the solids removed and the resulting solution treated with sodium carbonate or carbon dioxide, to precipitate calcium, strontium and barium carbonate followed by treatment with sodium hydroxide for precipitation of the hydroxide of magnesium, iron, aluminum and nickel. A portion of the muds containing the sulfate solids which have been removed following the first stage are returned, in the continuous process, to the first stage. However, the foregoing processes are directed to purification of brines containing substantial quantities of sulfate impurities and are therefore not readily adaptable to purification of brines which do not contain sulfate in substantial quantities. Moreover, in producing brines for use in processes in which the presence of high sulfate levels are not objectionable, these processes are unnecessarily complex and expensive. Finally, they do not achieve the low levels of strontium which has been found desirable in decreasing the formation of mercury butter in the subsequent mercury cell electrolysis of the purified brine.
Of interest also in removal of sulfate from raw brines is U.S. Pat. No. 3,753,900 (issued in 1973 to M. D. Moore).
Another process, herein termed the "Na.sub.2 CO.sub.3 /NaOH process", which is widely used to purify raw brines containing dissolved calcium, magnesium and strontium impurities, involves mixing the raw brine with an alkali metal carbonate, e.g. sodium carbonate, for precipitation of calcium and strontium carbonates, contacting the resulting brine containing carbonate solids with sodium hydroxide for formation of magnesium hydroxide solids, and removing the solids, generally by settling, to provide a liquor which, after optional filtration to remove any residual solids, may be either passed to a crystallizer for recovery of the pure sodium chloride salt or employed as purified brine in subsequent processing. Since the calcium and strontium carbonate solids are extremely difficult to remove by sedimentation even with the use of flocculants, this Na.sub.2 CO.sub.3 /NaOH process has the advantage of facilitating this sedimentation by adding sodium hydroxide to the brine subsequent to the addition of sodium carbonate. The magnesium hydroxide which forms precipitates in the form of flakes which incorporate the calcium and strontium carbonate crystals precipitated in the previous stage. Thus, the calcium carbonate and strontium carbonate crystals rapidly settle along with the magnesium hydroxide precipitate. After purification of raw brine by prior art Na.sub.2 CO.sub.3 /NaOH purification processes, the purified brine (generally containing from about 10 to 30 weight percent sodium chloride together with 1 to 300 ppm impurities) is passed to the electrolysis process. In a brine containing 25 weight percent sodium chloride, these impurities generally comprise from about 0.01 to 1 ppm iron, 0.1 to 30 ppm calcium, 0.1 to 10 ppm magnesium, 0.1 to 10 ppm aluminum and 0.6 to 10 ppm strontium. While concentrations of strontium lower than 0.5 ppm may be obtained by use of raw brine purification techniques such as ion exchange and evaporative crystallization techniques, disclosed in U.S. Pat. 3,954,580, such processes may not readily adapt to commercial purification facilities employing the Na.sub.2 CO.sub.3 /NaOH purification process wherein the raw brine is sequentially contacted with sodium carbonate and sodium hydroxide for impurity precipitation.
A similar process, herein termed the "NaOH/Na.sub.2 CO.sub.3 " process disclosed in I. Kanno and J. Yoshioka, "Brine Purification by a Sludge Circulation Process", Soda to Enso, 18(8), 274-81 (1967), 69 Chem. Abs. 61404 (1968), involves first adding sodium hydroxide to the raw brine followed by addition of sodium carbonate, passing the resulting liquor to a settling tank for removal of solids and recycle of a portion of the sludge from the settling tank to the vessel in which the raw brine is contacted with sodium hydroxide. The liquor from the settling tank is filtered and passed to an electrolytic cell for use therein. The Kanno et al. process, however, does not have the advantage of rapid settling of the calcium and strontium carbonate crystals due to forming of magnesium hydroxide solids subsequent to the forming of the calcium and strontium carbonate solids.
Other references disclosing brine purification processes are: M. L. Berman, et al., "Reactor-settling Tank for Purifying Brine for the Soda Industry", Vop. Proekt. Sodovykh Zavodov, No. 2, 55-9 (1971), 81 Chem. Abs. 27760g (1974); S. Yoshida et al., Japanese Kokai 74/66,600 (27 June 1974), 81 Chem. Abs. 130203b (1974); Y. Tamura, et al., Japanese Kokai 73/80,498 (27 October 1973), 80 Chem. Abs. 72564m (1974); I. A. Kuzin, et al., Sb. Tr. Kafedra Yad. Fiz. Radiats. Khim., Leningrad, Tekhnol. Inst. im. Lensoveta, No. 2, 3-16 (1971), 80 Chem. Abs. 125284d (1974); British Pat. No. 1,075,167 (issued in 1967 to J. H. Bendle, et al.), 67 Chem. Abs. 83529m (1967); and S. Pribicevic, et al., Glas. Hem. Drus., Beograd, 35(4-6), 363-8 (1970), 68 Chem. Abs. 131095k (1971).
Accordingly, there exists the need for a process which provides improved removal of strontium, calcium and magnesium impurities from brines.